The present invention relates to a method for hydraulic isolation determination of oilfield casings. More specifically, the interfaces between the various materials present in the borehole are interrogated using ultrasonic energies. The resulting signals are spatially filtered to reduce the sensitivity to the casing and increase the sensitivity of later (deeper) interfaces represented, for example, by formation reflections.
In a well completion, a string of casing or pipe is set in a wellbore and a fill material referred to as cement is forced into the annulus between the casing and the earth formation. The primary purpose of the cement is to separate oil and gas producing layers from each other and from water bearing strata. If cement fails to provide isolation of one zone from another, fluids under pressure may migrate from one zone to another, reducing production efficiency. Migration of water, in particular, produces undesirable water cutting of a producing zone and can possibly render a well non-commercial. Also, migration of hydrocarbons into aquifers is environmentally and economically undesirable. It is critical to determine whether the cement is performing its function to hydraulically secure hydrocarbon reservoirs. The term "good cement" indicates the adequate separation of zones by the cement, preventing fluid migration between the zones.
Cement failures occur in a variety of ways. For example, a complete absence of cement between the casing and the earth formation can occur. This is characterized as a gross cement failure and leads to rapid communication between zones intended to be isolated. Another type of failure arises when channeling occurs within the cement annulus between the casing and the formation. There are three commonly occurring types of channels. First, a channel which contacts the casing is referred to as a "near channel". Second, a channel which does not contact the casing is referred to as a "far channel" or a "buried channel". For a buried channel, the region between the channel and the casing is usually cement. And third, a channel occupying the entire space between the casing and the formation is referred to as either a "full channel" or a "traditional channel". All the channels described above are filled with fluids such as mud or gas and all are potential threats to hydraulic isolation.
Another condition which occurs, but which is not generally viewed as a cement failure, is known as micro-annulus. This condition occurs when the cement that has filled the annulus is not properly bonded to the casing resulting in a very narrow fluid-filled annulus immediately outside the casing. This annulus is very small and does not affect fluid communication between layers, effectively preserving the hydraulic security function of the cement.
A completed well includes a number of interfaces at the junctures of the differing materials within the wellbore. A first interface exists at the juncture of the fluid in the casing and the casing itself. The casing is referred to as a first material and is typically made of steel. A second interface is formed between the casing and a second material adjacent to the exterior of the casing. If cement is properly placed between the casing and the formation, providing hydraulic isolation, the second interface exists between the casing (first material) and the cement (second material). Further, a third interface exists between the cement and a third material which is the earth formation.
Imperfect cementing operations can result in a variety of interface conditions. A channel contacting the casing results in the second interface being between the casing (first material) and a fluid (second material). In this case, the third interface is formed between a fluid (second material) and the earth formation (third material) where a full channel exists. Alternatively, the third interface is formed between a fluid (second material) and the cement (third material) where a near channel exists. A channel not contacting the casing, results in the second interface being between the casing (first material) and the cement (second material) and the third interface being between the cement (second material) and a fluid (third material). Existence of a buried channel causes a potential lack of hydraulic isolation.
The problem of investigating the fill material or cement outside a casing with a tool located inside the casing has lead to a variety of cement evaluation techniques using acoustic energy. Currently, it is believed that the most significant parameter in predetermining the quality of a cement job is the centralization of the casing in the borehole. Knowing the centralization before the cementing operation would be advantageous. The opportunity exists to make this measurement while the annulus is filled with a fluid, i.e., either before cementation or after cementation before hydration. Also, this measurement could be repeated over the cure time of the cement to provide time-lapse data.
Schlumberger Technology Corporation (assignee) makes high resolution cement evaluation measurements with the Cement Evaluation Tool (CET.TM.) and the Ultrasonic Imager (USI.TM.). Both of these tools perform the same physical measurement. The casing-thickness resonance is excited, by radially propagating energy from a pulse-echo transducer (in the fluid), and the decaying, resonant "tail" of the received waveform is analyzed. For both of these tools, each spatial location measured is analyzed independently of any information from other locations.
The "head" of the received waveform is energy reflected from the inner surface of the casing. It is the earliest and highest amplitude portion of the waveform. It contains no information of materials beyond the casing. The decaying, resonant tail of the received waveform is predominantly energy which has resonated within the casing and, therefore, contains information about the casing itself and about the materials in direct contact with the casing (mud or cement). Energies reflecting from structures not contacting the casing, such as fluid-channels or the formation, are also contained within this tail, but are typically much less energetic than the dominant steel resonances. This makes the direct measurement of these later structures very difficult as the signal in the measurement window is largely controlled by changes in the casing (e.g., local thickness) or by changes in the casing/cement interface (e.g., local disbonds).
The processing schemes employed in the USI.TM. attempt to minimize the effects of later reflections from objects not contacting the casing to most accurately measure the casing thickness and the cement impedance.
In general, any acoustic measurement technique used on a casing-cement-formation system will result in received energy which is dominated by the casing reflections. The reflections from typical, outer structures will be relatively weaker signals.